1. Field of the Invention
The invention relates to an activation device for a piston/cylinder unit having a selectively inhibited movement, in particular for a gas spring which can be selectively inhibited including a cylinder, a piston on a piston rod, the piston dividing the cylinder into two working spaces filled with a hydraulic medium, and a flow connection system between the working spaces including an on/off valve.
2. Description of the Prior Art
Gas springs which have a selectively inhibited movement are employed, for example, in chairs which have a seat back which is spring-supported and can be rigidly held by an inhibiting function of the gas spring. A chair user does not, however, continuously adopt the same posture on the chair. The seat back position can be very easily matched to the instantaneous sitting position by an activation device. Furthermore, the user seated in the chair may also lean forward so that the seat back has no contact with the user's back. If the activation device for the gas spring on the seat back is actuated when the user is leaning forward, the seat back is accelerated by the gas spring toward the user's back and may impact the user's back. This operational behavior is unidesireable. DE 69 35 911 U1 describes a gas spring which has a divided activation push-rod, a spring being arranged between the two activation push-rod sections. Slower opening of the inhibiting valve within the gas spring is achieved by using the spring
A similar approach is described in DE 36 15 688 A1 in which a valve cross section changes over the stroke path of the activation push-rod. The throttling associated with this only permits the piston rod to extend slowly at the beginning of the stroke path of the valve and then increasingly rapidly near the end of the stroke path. Although an unpleasant contact with the seat back is weakened by both of the above mentioned prior art types of gas springs, the undesireable behaviour is not reliably prevented.
GB 2 304 036 (GB '036) discloses a seat with an adjustable back and describes an activation device which has two switching inputs. The activation push-rod of the activation device may be activated by a Bowden cable. In addition, the seat support is likewise in effective connection with the activation device such that, in the case of an unloaded seat support, the uninhibited operating function is always adopted on the gas spring independently of the switching condition of the Bowden cable. An activation appliance, which acts directly on the activation push-rod and which is embodied as a rocker arm is used for this purpose. An essential feature of the described activation device is that the seat panel for detecting the loading condition is required to carry out a pivoting motion which imposes a limitation on the chair design. Comfortable chairs often feature a seat inclination adjustment. However, this type of seat inclination adjustment would have a substantial influence on the function of the activation device disclosed by GB '036.